1. Field of the Invention
The present invention relates to a method of manufacturing a silicon substrate with a recess, a method of manufacturing an ink jet head, a silicon substrate with a recess, and an ink jet head comprising a silicon substrate in which a recess forms a diaphragm. The present invention relates more particularly to an ink jet head for use in a printer, facsimile machine, or other printing device.
2. Description of the Related Art
Ink jet heads used in printers, facsimile machines, and other types of printing devices print by ejecting ink drops onto the printing medium, and various methods have been proposed for the ink ejection mechanisms. One such method uses a heater to vaporize the ink, creating a pressure bubble whereby an ink drop is ejected. Another method applies voltage to a piezoelectric element, which is affixed to the ink chamber in which the ink is held, to expand and contract the ink chamber (increase and decrease the internal volume) and thereby cause an ink drop to be ejected. Yet another method use electrostatic force (an electrostatic actuator) to change the volume of the ink chamber in which the ink is held, and thereby eject an ink drop.
The ink jet heads used in these various methods are manufactured using precision semiconductor processing techniques. For example, an ink jet head that ejects ink drops by means of electrostatic force is achieved by forming a recess (diaphragm) in a silicon substrate, arranging an electrode opposed to the diaphragm with a specific gap therebetween, and inducing an electrostatic force between the recess (diaphragm) and electrode to displace the recess (diaphragm) and thereby change the internal pressure of the ink chamber to eject an ink drop from an ink nozzle. This recess (diaphragm) and the member on which the electrode is disposed are referred to as "opposing members."
The ink chamber is formed by bonding a second substrate of silicon, glass, or other material to the silicon substrate in which the recess (diaphragm) is formed such that the recess (diaphragm) is covered by the second substrate. The recess (diaphragm) of the first silicon substrate thus forms one wall of the ink chamber.
Precision semiconductor processing technologies are used to produce the recess (diaphragm) in such silicon substrates. That is, an etchant-resistant material (mask) is formed on the silicon substrate for shaping the recess (diaphragm), and the substrate is then etched to produce the recess (diaphragm).
Thermal oxidation of a silicon substrate produces a thermal oxidation film on the substrate surface, and this thermal oxidation film is typically used as a mask.
A protective thermal oxidation film is also typically formed on the surface of the silicon layer in which the recess (diaphragm) is formed before bonding with the opposing substrate as a means of improving wettability with the ink and preventing corrosion of the silicon by the ink.
Methods of manufacturing such ink jet heads are taught by the present inventor in Japan Unexamined Patent Application Publication (kokai) H3-79350 and H6-71882.
In general, there are two methods for forming a thermal oxidation film, wet oxidation and dry oxidation. Dry oxidation is a slower process for film formation (film formation rate), but results in a dense oxidation film of good quality. Wet oxidation produces a film that is not as dense and inferior in quality compared with the dry oxidation film, but the film formation rate is faster.
A thermal oxidation film is formed to protect the ink channel walls and diaphragm from dissolution by ink. When the film is formed by wet oxidation, the inferior quality of the resulting thermal oxidation film results in relatively poor ink resistance. The silicon substrate therefore becomes more susceptible to corrosion and dissolution by ink. This is also true when the second substrate is made from silicon and the thermal oxidation film formed thereon is achieved by wet oxidation.
The lower density of thermal oxidation films formed by wet oxidation also means that sufficient electrical isolation may not be achieved. If this method is then used to produce an electrostatically driven ink jet head, the electrostatic charge produced between the recess (diaphragm) and electrode can discharge and damage the recess (diaphragm).
The fast film growth rate of the wet oxidation process also makes it difficult to control the film thickness with high precision. Variations in film thickness therefore result, the electrostatic attraction characteristic of the recess (diaphragm) is degraded, and the electrostatic attraction force, in particular, can drop. This degradation of the electrostatic attraction characteristic can prevent ink drops from being ejected with appropriate volume, and can therefore degrade print quality.
It is possible to improve the corrosion resistance and insulation properties of the thermal oxidation film formed by wet oxidation by simply increasing the film thickness. However, when the film thickness is increased, the electrostatic force produced between opposing members drops, and electrostatic attraction drops accordingly. It is therefore difficult to reduce power consumption and device size.
It is also obviously possible to form both the protective thermal oxidation film and the thermal oxidation film used as a mask by means of a dry oxidation process. The slow film formation rate of the dry oxidation process, however, reduces the productivity of ink jet head manufacturing, and therefore leads to increased ink jet head cost.